Background
Land is an indispensable resource for the most essential human activities: it provides the basis for agriculture and forest production, water catchment, recreation, and settlement. The range of uses that can be made of land for human needs, is limited by environmental factors including climate, topography and soil characteristics, and is to a large extent determined by demographic, socioeconomic, cultural, and political factors, such as population density, land tenure, markets, institutions, and agricultural policies.
The Food and Agriculture Organization of the United Nations (FAO) with the collaboration of the International Institute for Applied Systems Analysis (IIASA), has developed a system that enables rational land-use planning on the basis of an inventory of land resources and evaluation of biophysical limitations and potentials. This is referred to as the Agro-ecological Zones (AEZ) methodology.
Recent availability of digital global databases of climatic parameters, topography, soil and terrain, vegetation, and population distribution has called for revisions and improvements in calculation procedures and in turn has allowed for expanding assessments of AEZ crop suitability and land productivity potentials to temperate and boreal environments.
Methodology
The AEZ methodology follows an environmental approach; it provides a standardized framework for the characterization of climate, soil and terrain conditions relevant to agricultural production. Crop modeling and environmental matching procedures are used to identify crop-specific limitations of prevailing climate, soil and terrain resources, under assumed levels of inputs and management conditions. This part of the AEZ methodology provides maximum potential and agronomically attainable crop yields for basic land resources units (usually grid-cells in the recent digital databases).
The AEZ computations were completed for a range of climatic conditions, including a reference climate (average of period 1961-1990), individual historical years of 1960 to 1996, and scenarios of future climate based on the published outputs of various global climate models. Hence, the AEZ results consistently quantify impacts on land productivity of historical climate variability as well as of potential future climate change.
The FAO/UNESCO Digital Soil Map of the World (DSMW) has been made the reference for constructing a land surface database comprising of more than 2.2 million grid-cells at 5’ latitude/longitude within a raster of 2160 rows and 4320 columns. On the input side, the key components of the database applied in AEZ include the FAO DSMW and linked soil association and attribute tables, a slope distribution database, and a layer providing distributions in terms of eleven aggregate land-cover classes derived from a global 30 arc-seconds latitude/longitude seasonal land cover data set. On the output side, many new data sets have been compiled at grid-cell level and have been tabulated at country and regional level, including general agro-climatic characterizations of temperature and moisture profiles, and time-series of attainable crop yields for major food and fiber crops.
The information contained in these data sets forms the basis for several further AEZ applications. Examples are: the quantification of land productivity, the estimation of extents of land with rain-fed or irrigated cultivation potential, the occurrences of environmental constraints to agricultural production, the identification of potential ‘hot spots’ of agricultural conversion, and the possible geographical shifts of agricultural land potentials as result of changing climate. Finally, the results of AEZ land productivity assessments provide a spatially explicit and agronomically sound basis for applications of multi-criteria optimization of land resources use and development.
Findings
The 1998 Revision of the United Nations medium variant population projection indicates an increase of world population to about 8.9 thousand million by the year 2050, with a possible range of 7.3 to 10.7 thousand million. Most experts agree that through full and adequate application of modern agricultural technology, the world's land resources could provide sufficient food, fiber, animal feed, biofuel and timber for such a world population. In practice, however, there will very likely be acute land shortages in some regions, especially in several developing countries. The AEZ procedures and applications have been used to provide an up-to-date environmental assessment of global food and fiber prospects.
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1
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The AEZ results confirm that
the Earth’s land, climate and biological resources are sufficient to meet
the needs of food and fiber of future generations, and more in particular
for a world population of 8.9 thousand million, as projected for the year
2050 by the UN medium variant. |
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Despite this affirmative aggregate
picture, there are also reasons for profound concerns. Several regions
exist, where the rain-fed cultivation potential has already been exhausted,
as for example is the case in parts of Asia. Land degradation, if continuing
unchecked, may exacerbate regional land scarcities. Concerns for the environment
may prevent some resources from being developed for agriculture. Global
warming may alter the condition and distribution of land suitable for
cropping. In addition, socioeconomic development may infringe on the current
agricultural resource base for want of rapidly expanding industrial and
service sectors. |
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3
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On the basis
of currently available global soil, terrain and climate data, the AEZ
approach estimates that 10.5 thousand million hectares of land, i.e.,
more than three-quarters of the global land surface (excluding Antarctica),
suffer rather severe constraints for rain-fed crop cultivation. Some
13 percent is too cold, 27 percent is too dry, 12 percent is too steep,
and about 65 percent are constrained by unfavorable soil conditions
(multiple constraints coincide in some locations).
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Various ways are available for estimating the extent of land with rain-fed cultivation potential for rain-fed crops. Any quantification depends on a variety of assumptions: the range of crop types considered, the definition of what level of output qualifies as acceptable, the social acceptance of land-cover conversions (in particular of forests), and the assumptions on what land constraints may be alleviated with modern inputs and investment. Hence, our estimates range from 3 thousand million ha (land very suitable and suitable for major cereal crops, under high inputs and mechanization, outside current forest areas) to 3.3 thousand million ha (land very suitable, suitable or moderately suitable for at least one of the AEZ crop types, within or outside current forest areas). The results presented in this study are based on the following calculation procedures for each grid-cell:
The total extents obtained in this way for each grid-cell, referred to as mixed level of inputs, were calculated for rain-fed and rain-fed plus irrigated conditions. When considering all modeled Global AEZ crop types (excluding silage maize, forage legumes and grasses), mixing all three input levels, and assuming no restrictions for land-cover conversion, we conclude that about one-quarter of the global land surface (excluding Antarctica) can be regarded as potentially suitable for crop cultivation. In developed countries about one-fifth comprises of land with rain-fed cultivation potential. In developing countries it amounts to about 28 percent. This estimate, based on a rather generous definition of land with cultivation potential, is twice the area estimated as actually in use for cultivation in 1995-97 (FAO, 2000; Table 4.7, p. 104).
Land with rain-fed cultivation potential for major food and fiber crops (million ha) Nevertheless, there are several regions where the rain-fed cultivation potential is nearly fully exhausted or has already been exceeded. If forests were to be maintained, then 2.55 thousand million ha would qualify as land with cultivation potential, of which 1.94 thousand million ha are adjudged very good or good suitability.
Rain-fed
cultivated land in 1994-96 (Source: FAOSTAT)
and rain-fed cultivation potential for major food and fiber crops (million
ha) |
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| 5 |
Despite recognizing that statistics of land in cultivation are likely to be underestimating actual use in developing countries by some 10 to 20 percent (see FAO, 2000), the Global AEZ results indicate that there is still a significant potential for conversion to arable use in Africa and South America, including from current forest areas. In other regions this potential is either exhausted (e.g., Asia) or unlikely to be exploited for agriculture under current and expected future conditions (i.e., Europe, North America and Oceania).
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| 6 |
By looking at the full range of crop types, without consideration of the demand for different products, we may overestimate the useful extents of land with cultivation potential. When restricting the considered crop types to the three major cereals, namely wheat, rice, and grain-maize, and allowing for nonagricultural land uses, an estimate of about 2.4 thousand million ha of land with rain-fed cultivation potential was obtained. Of these, 1.5 thousand million ha were found in developing countries and 0.9 thousand million ha in developed regions.
Land with
rain-fed cultivation potential for wheat, rice or grain-maize (million
ha) |
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| 7 |
Combining AEZ results with
spatial land cover data, the extent of land with cultivation potential
presently under forest ecosystems was estimated. About 237 million hectares
of the area classified as forest ecosystems was assessed as very suitable
or suitable for cultivation of wheat, rice or grain-maize at high level
of inputs. On the other hand, the analysis shows that globally almost
85 percent of forest ecosystems are considered not suitable or at best
marginally suitable for cereal cultivation.
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| 8 | The study highlights the
uneven distribution of land and climate resources. We use population estimates
of 1995 and a suitability index SI
SI = 0.9 * VS + 0.7 * S + 0.5 * MS + 0.3 * mS VS = very suitable, S = suitable, MS= moderately suitable, mS = marginally suitable (a weighted sum of different land suitability classes optimizing over all AEZ food crops and for mixed input levels) to compare per capita availability of land with crop cultivation potential across regions, both with and without consideration of areas classified as dominantly forests. The variables SI_N and SI_F refer respectively to the index numbers calculated for non-forest areas (SI_N) and within forest areas (SI_F). For total availability (SI_N plus SI_F) the world average is 0.45 suitability index units per person, ranging between 0.12 units in Asia to 3.2 units per person in Oceania.
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| 9 |
Assuming availability of water resources, but limiting the analysis to soil conditions indicating presence of water (such as Gleysols and Fluvisols), some 65 million hectares, i.e., only about 1.8 percent of arid and hyper-arid zones, were assessed as prime land for cereals under irrigation, which in turn equates to less than 3 percent of total prime land for cereals. The results suggest that irrigation is more important in providing stable water supply in areas of climatic variability rather than for bringing land in hyper-arid and arid regions into cultivation.
Distribution of land with cultivation potential by impact classes, showing increase in potential output with full exploitation of irrigation. Full exploitation of all
potential irrigable land would increase the global gross extent of suitable
land for cereals by 6 to 9 percent. The impact of irrigation is more
pronounced on the increase of potential production than on potential
area. The global cereal production potential would increase by 30 to
40 percent. |
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| 10 |
The application of a set
of temperature and rainfall sensitivity scenarios revealed a modest
increase of cultivable rain-fed land for temperature increases up to
2°C on global scale. With a higher temperature increase alone, extents
of cultivable rain-fed land start to decrease. When both temperature
and rainfall amounts increase, the extents of cultivable rain-fed land
increase steadily. For example, a temperature increase of 3°C paired
with a rainfall increase of 10 percent would lead globally to about
4 percent more cultivable rain-fed land. In the developed countries
this increase is even markedly higher; it exceeds 25 percent. Contrariwise,
for developing countries there would be a decrease of 11 percent.
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